We have shown that mammals share with submammalian species a functionally significant 5-tiered stratification of the IPL. This 5- tiered stratification, dominated by narrowly stratified amacrine and ganglion cells, is obscured, however, in ordinary histological preparations of mammalian retina by the superimpositionof a "simpler" system of bi-sublamination, which coincides with the separate distribution of the synaptic terminals of "flat" and "invaginating" cone bipolars in the outer 1/3 and inner 2/3 of the IPL, respectively. The differing connections of the two cone bipolars with cone photoreceptors suggest that one is concerned with "lightness" and the other with "darkness". Correspondingly, the two sublaminae of the IPL may also be concerned with "lightness" or "darkness". We have found three types of ganglion cells which branch either in the inner 2/3 or in the outer 1/3 of the IPL, and in the cat these constitute a majority of the ganglion cells: the large and small "tufted" cells and the large- bodied "radiate" cells. The dendritic branching patterns of "tufted" and "radiate" cells have been analyzed in Golgi preparations with the aid of a PDP-12 image-processing computer in order to give a quantitative basis for the ganglion cell classification. Small- and medium-bodied radiate ganglion cells are of at least 7 types in cat and include the bistratified and "E" type ganglion cells, typical of nonmammalian retinas. Preliminary studies on the synaptic connections of different types of retinal ganglion cells will be continued with the object of working out the complete "wiring diagram" of mammalian retina.